One of the major problems in the development of separators for lithium ion or metal batteries (LIBs, LMBs) is to optimize both ionic conductivity (σ) and lithium ion transference number, tLi+, the fraction of the charge carried by the electroactive Li+ ions. For tLi+→1, the anions do not move, and therefore concentration polarization does not develop. Concentration gradients adversely affect cell performance by contributing to the growth of dendrites, which are branched or needle-like structures that form instead of desirable flat, uniform Li0 deposition. Dendrites can break off, causing a decrease in energy density, or span the cell separator causing internal shorting, heating, thermal runaway and catastrophic cell failure. Dendrites are particularly a concern for lithium metal batteries (LMBs), but also occur in lithium ion batteries (LIBs), since Li0 metal rather than intercalated Li+ can deposit on the anode in LIBs during fast charging or at low temperatures (Armand and Tarascon, Nature 2008, 451, (7179), 652-657). The transition from graphitic to metallic lithium anodes would enhance energy density 10-fold, and metallic lithium anodes would enable the use of unlithiated materials such as sulfur or air to replace intercalation cathodes for lithium/sulfur or lithium/air batteries (Bruce, et al., Nature Materials 2012, 11, (1), 19-29; Grande, et al., Advanced Materials 2015, 27, (5), 784-800) with improved energy density. In these cases, the safety issues associated with dendrite growth when using metallic Li0 are a major concern.
There has been extensive theoretical and experimental research on the prevention of dendritic growth, including mechanical inhibition (Monroe and Newman, Journal of the Electrochemical Society 2005, 152, (2), A396-A404) and limiting concentration gradients that result in anion depletion near the anode (Chazalviel, Phys. Rev. A 1990, 42, (12), 7355-7367). The latter can be avoided by the use of electrolytes with high ionic conductivities and low anion mobilities, i.e. single ion conductors (SICs). Concentration gradients are avoided in polymer single ion conductors (SICs) by covalent attachment of the anions to the polymer backbone so that tLi+→1. However, conductivities of polymer SICs have remained low (σ<10−6 S/cm). In polymer electrolytes, the cation motion is believed to be coupled to the backbone dynamics, so attempts to increase conductivity have included incorporating flexible chains with low glass transition temperatures (Tgs) such as polydimethylsiloxanes. As in the case of bi-ionic conductors, the conductivity increases as the electron withdrawing groups of the anion increase, and as the negative charge becomes more delocalized. Thus, replacement of carboxylate, phosphate and sulfonate anions with the LiTFSI-like anion in lithium[(4-styrenesulfonyl)trifluoromethane-sulfonyl)imide] (Meziane, et al., Electrochimica Acta 2011, 57, 14-19; Feng, et al., Electrochimica Acta 2013, 93, 254-263) and poly[(4-styrenesulfonyl) (trifluoromethyl (S-trifluoromethylsulfonylimino) sulfonyl) imide] (Ma, et al., Angewandte Chemie-International Edition 2016, 55, (7), 2521-2525) have resulted in the best ion conductivities to date.
There is also evidence that dendrites can be blocked in liquid electrolytes with cellulose-based separators with small pores (50-100 nm) (Yu, et al., ACS Energy Letters 2016, 1, (3), 633-637), or in solid or gel electrolytes if channels are formed in the matrix that are smaller than a critical dimension, i.e. two small for infiltration of dendrites (Tu, et al., Advanced Energy Materials 2014, 4, (2), 6; Tu, et al., Accounts of Chemical Research 2015, 48, (11), 2947-2956). This was also suggested to be the case for liquid electrolytes encapsulated within close-packed hollow, porous silica spheres (Zhang, et al., Nano Letters 2015, 15, (5), 3398-3402). Modelling studies of electrodeposition also predicted that stability (i.e. lack of dendrites) was increased in electrolytes when a small fraction of the anions were immobilized due to a reduction in the electric field at the Li0 electrode (Tikekar, et al., Journal of the Electrochemical Society 2014, 161, (6), A847-A855).
There remains a need in the art for single ion conductors with high ionic conductivity and lithium ion transference number. The present invention fulfills this unmet need.